How can I change all files belonging to one user to another user?

I think the --from flag on chown command is probably the easiest way.

chown --from=oldguy newguy * -R

Something like

find /decompressed-backup-dir -uid 1050 -exec chown newuser:newgroup {} +

Adding to the answer by SiteKickr, chgrp does not have the --from argument, but you can achieve the same with chown by omitting the user.

Example:

chown -R --from=:currentgroup :newgroup /some/directory